1. Field of the Invention
The present invention relates to drill bits with rotating cone cutters having a plurality of annular rows of inserts projecting therefrom.
2. Description of the Prior Art
The use of insert-type drill bits for drilling extremely hard formations has been known in the oil industry for many years. Such drill bits conventionally comprise two or three legs welded together to form a unitary structure. The one end of the structure forms a threaded shank for attachment to the lower end of a drill stem. The lower end of each leg includes an inwardly facing spindle for rotatively supporting a cone cutter having a plurality of annular rows of cutting inserts mounted thereon. Upon rotation of the drill bit, the cone cutters are adapted to substantially roll on the bottom of the hole to be drilled. During this rolling movement, the cutting inserts are disposed to contact and disintegrate the earth formation at the hole bottom. The chips and cuttings, dislodged from the earth formation, are flushed away by the drilling fluid passing across the face of the bored hole.
In a conventional 2- or 3-cone drill bit, a row of cutting inserts are provided at the heel of each cone to cut the bore hole to gage while other annular rows are spaced inwardly on the cones to cut the remainder of the bore hole bottom. In such conventional drill bits, the cones are configured with rounded heels to enable the gage row to have a shorter extension than the adjacent rows thereby alleviating the wear of the gage rows during the drilling operation. However, in such a configuration, the rounded heels and the lesser extending gage rows engage the hole bottom last thereby causing the drill bit to follow any deviation already existing in the bore hole. As a result, the drill bit would veer from its desired direction of travel.
A previous solution to this problem is found in U.S. Pat. No. 3,452,831. In the drill bit described in that patent, the gage row of inserts extends out radially further than the adjacent rows of inserts. However, any beneficial results of the gage row is negated by the reaming row of inserts located directly behind the gage row. The shortcoming with the reaming row is that it takes away from the support mass of the cone heel. As a result, the limited support of the gage row of inserts only allows for a point contact with the gage of the bore hole. Anything more than this minimal contact with the gage row would cause the gage inserts to break off from the cones.